The debate about how to get better mobile phone coverage to remote villages and country roads has today driven straight into the fraught issue of national security and surveillance.

Theresa May, the home secretary, has written to other ministers warning them that a plan by her political rival, Sajid Javid, the culture secretary, to force the UK’s four networks to share their signal in the shires will leave the country open to terrorist attacks.

The prime minister, who was unable to make calls while on holiday in Cornwall,is keen to find a solution. Rural residents are desperate for better broadband services, so a pre-election pledge to bring more homes within reach of a simple voice call would seem like a way to swing a few more votes.

There is no doubt that improvements to mobile and broadband infrastructure bring economic benefits. But operators were already resisting Javid’s proposals on commercial grounds. Now that May has waded in on their side, will security concerns derail the project?

Today, 1.5m people live in areas where there is a signal from one or two, but not all four, mobile phone networks. These partial “not-spots” affect 20% of the UK landmass, 3% of premises, 10% of A roads and 16% of B roads. Investment by O2 and Vodafone, which have agreed to share masts, will reduce partial not-spots to 13% of the landmass soon. But how to bring that number to zero?

Javid’s proposals, released for consultation on Wednesday, offer three solutions. The most controversial is called national roaming – allowing customers to jump from one network to another, depending which has the best or only available signal. It is similar to international roaming, when deals between operators mean we can use foreign networks while travelling without having to subscribe.

This solution would tackle all the partial not-spots, but cost up to £400m over 10 years in new IT systems, testing and equipment, plus potentially higher rental costs for mast sites. There could be higher charges for customers too, as networks buy minutes from each other. Another proposal is for a single subscription giving access to all four networks, sold for example by Tesco Mobile or Virgin Mobile. Finally, the operators are proposing to share more masts, mast sites, and equipment. Javid’s advisers reckon their proposals would only reduce not-spot areas to 3% of the landmass.

May’s letter, which is light on technical detail, says national roaming would make life more complicated for the police and security services. One operator source dismissed her arguments as a politically motivated “red herring”. Others, however, warn national roaming would make it almost impossible to trace the location of a call in real time in rural areas, harder to tap calls, and more complicated to retrospectively retrieve metadata – information such as who was calling whom, where and when.

To trace caller location in real time, for example in a kidnapping, the work is done manually and requires a team of people at the network’s data centre. They track the call from mast to mast, while the police listen in over the phone.

But national roaming would not be seamless – meaning the caller would be cut off each time a call changed masts. When they dialled again, they could be routed through a different network. This could happen every two or three minutes in a moving vehicle.

If the police were tracking location through Vodafone, because that was the caller’s usual network, they would lose contact once was the call was routed through an EE mast, for example. To be sure of following a rural caller jumping between three or four networks, the police would have to set up simultaneous tracking operations. The same issues arise when listening in to calls.

For retrospectively retrieving information on calls, text messages and web browsing – the metadata showing where and when a call was made, and to whom – police would need to issue requests to all four networks. According to an operator source: “You go from a position where you have a single source of information – one network – to forcing police and security services to assemble a jigsaw.”

Such blanket issuing of warrants would seem to take us in the opposite direction from the proportionate and targeted surveillance many would prefer. But coverage does need to be improved. In Scotland, a quarter of the country has no 2G (voice) signal. And the real economic benefits come with 4G mobile internet, an even scarcer resource outside urban areas.

National roaming may or may not be the right solution. But the government has already offered the networks a cut in the rental fees they pay for some of their spectrum if they agree to spend more on masts. So varying their licences to mandate wider coverage, not only for voice but for internet connections, is financially justifiable. It would also ensure the consumer benefits while leaving the networks to decide for themselves how to get the job done.